Index-matched phosphor scintillator structures

ABSTRACT

Scintillator structures are disclosed in which the phosphor is embedded or suspended in an optically transparent matrix which is selected or adjusted to have an index of refraction which is approximately equal to that of the phosphor embedded therein at the wavelength of the light emitted by the phosphor. In accordance with one embodiment of the invention, BaFCl:Eu is embedded in a matrix formed by the polymerization of 2-vinyl napthalene and vinyl toluene. The scintillator structures of the present invention provide superior optical coupling to photoelectrically responsive devices. Also disclosed are methods for manufacturing index-matched phosphor scintillator structures.

This application is a continuation-in-part of application Ser. No.863,876 filed Dec. 23, 1977, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to scintillator structures and methods formanufacturing such structures. More particularly, this invention relatesto a method of enhancing the escape of visible wavelength radiation fromthe scintillator structure by matching the index of refraction of thephosphor particles embedded therein with the index of refraction of theoptically transparent matrix in which the phosphor is embedded.

In general, a scintillator is a material which emits electromagneticradiation in the visible spectrum when stimulated by high energyelectromagnetic photons such as those in the x-ray or gamma-ray regionsof the spectrum, hereinafter referred to as supra-optical frequencies.Thus, these materials are excellent choices for use as detectors inindustrial or medical x-ray or gamma-ray equipment. In most typicalapplications, the light output from scintillator materials is made toimpinge upon photoelectrically responsive materials in order to producean electrical output signal which is in direct relation to the intensityof the initial x-ray or gramma-ray bombardment.

Scintillator materials comprise a major portion of those devices used todetect the presence and intensity of incident high energy photons. Theother commonly used detector is the high pressure noble gas ionizationdevice. This other form of high energy photon detector typicallycontains a gas, such as xenon, at a high pressure (density), whichionizes to a certain extent when subjected to high energy x-ray orgamma-ray radiation. This ionization causes a certain amount of currentflow between the cathode and the anode of these detectors which are keptat a relatively high and opposite polarity from one another. The currentthat flows is sensed by a current sensing circuit whose output isreflective of the intensity of the high energy radiation. Since the highpressure noble gas detector operates on an ionization principle, afterthe termination of the irradiating energy, there still persists thepossibility that a given ionization path remains open through which anundesirable leakage current may pass. Hence, these detectors arepeculiarly sensitive to a form of "afterglow" or persistence similar tothat found in certain scintillating phosphors. This persistence resultsin the blurring in the time dimension of the the information containedin the irradiating signal.

In general, it is desirable that the amount of light (visible or nearvisible wavelength) output from these scintillators be as large aspossible for a given amount of x-ray or gamma-ray bombardment. This isparticularly true in the medical tomography area where it is desiredthat the energy intensity of the x-ray be as small as possible tominimize the danger to the patient. For this reason the phosphorscintillator should have a good luminescent efficiency.

Another important property that scintillator materials should possess isthat of a short afterglow or persistence. This means that there shouldbe a relatively short period of time between the termination of the highenergy radiating excitation and the cessation of light output from thescintillator. If this is not the case, there is resultant blurring, intime, of the information-bearing signal. Furthermore, if a rapidscanning is desired, as it is in certain computerized tomographicapplications, the presence of the afterglow tends to severely limit thescan rate, thereby rendering difficult the viewing of moving bodilyorgans, such as the heart or lungs.

A scintillator body or substance, in order to be effective, must be agood converter of high energy radiation (that is, x-rays andgamma-rays). Typically, present scintillator bodies consist of aphosphor in a powder or crystalline form. In this form, the useful lightthat is produced upon high energy excitation is limited to that which isgenerated in the surface regions of the body and that which can escapethe interior of the scintillator body. This escape is difficult due tomultiple internal reflections, each such reflection further attentuatingthe amount of light externally available by allowing considerably moretraversal of phosphor than desired. Thus, it is necessary that not onlythe phosphors themselves have a good luminescent efficiency but it isalso necessary that the light output be available for detection.

In the copending application of Dominic A. Cusano and Jerome S. Prener,Ser. No. 853 086, assigned to the same assignee as this invention, thereis described distributed phosphor scintillator structures in which thephosphor is either embedded in an optically transparent matrix or inwhich the phosphor occurs in a layered structure with alternating layersof phosphor and optically transparent laminate material. This copendingapplication is incorporated by reference herein. In this prior copendingapplication there is still the problem that light rays generated withinthe scintillator body are refracted and reflected amongst the embeddedphosphor particles as a result of the fact that there is a difference inthe index of refraction between the phosphor particles and the matrixmedium in which they are embedded. This mismatch results in a certainloss of efficiency as measured by light energy escaping the scintillatorbody.

The term "optical transparency" as used above and hereafter refers tothe transparency of the scintillator body or material at or near thewavelength of light emitted by the phosphor or by a single or finalwavelength conversion material in the embodiment wherein more than onewavelength conversion material is added. It is to be further noted thatthe index of refraction of light transmissive materials is in generaldependent upon the wavelength of the transmitted light. Thus, themismatch of indices of refraction mentioned above is a mismatch which isdependent upon the wavelength of light under consideration.

In particular, in the medical tomography area, where the intensity ofx-radiation is modulated by the body through which it passes, and whichmodulated radiation is then converted into electrical signals, it isimportant to have x-ray detection devices which have as good overallenergy conversion efficiency as possible. For devices with lowefficiency, a higher flux of x-ray radiation must be applied to producethe same light and electrical output from the overall scintillationdetector system. In the context of medical tomography, this means thatsuch systems have a low signal-to-noise ratio.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a phosphor isembedded in an optically transparent matrix which has been selected oradjusted to have an index of refraction approximately equal to the indexof refraction of the phosphor at or near the wavelength of the opticaloutput of the phosphor. The matrix material in which the phosphor isembedded is either a solid, or a liquid in which the phosphor issuspended. In accordance with one embodiment of the present invention,the phosphor is mixed with two monomers and the resulting mixture isthen polymerized in a heat treatment process to form a solidscintillator body. In accordance with another embodiment of the presentinvention, the phosphor is mixed with a pulverized polymer and is heatedunder pressure to form an optically transparent scintillator body. Instill another embodiment the phosphor is mixed with a solution in whichthe polymer has been dissolved; the solution is then freeze-dried toremove the solvent; the resulting powder is pulverized and then heatedunder pressure to form an optically transparent scintillator body.

In the scintillator bodies of the present invention, the boundariesbetween the phosphor particles and the matrix in which they are embeddedor suspended are practically invisible to the light rays generated byabsorption of a high energy photon. Hence, the resulting light pathsfrom the absorption event to the exterior of the scintillator body arerelatively straight with little reflection or refraction at theboundaries of the phosphor particles.

Accordingly, it is an object of this invention to provide a transparentmatrix surrounding and supporting scintillator phosphor particles whichis matched to the index of refraction of the phosphor particles.

It is therefore, a further object of this invention to providescintillator bodies having a superior optical output.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation sectional view illustrating the opticalbehavior of prior art scintillator bodies.

FIG. 2 is a side elevation sectional view of the scintillator body ofthe present invention illustrating the effect of the high energyabsorption event.

FIG. 3 is a graph of the indices of refraction as a function of lightwavelength for a particular phosphor and a particular index-matchedtransparent matrix material.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the operation of a scintillator body composed of apowder or polycrystalline phosphor material. In this prior art form ofscintillator body a high energy gamma-ray or x-ray photon 12 is absorbedat absorption site 11 inside the scintillator body and is converted intomultiple lower energy optical wavelength photons in the visible or nearvisible (ultraviolet or infrared) regions depending upon the phosphor 13used. Because of the difference in the index of refraction between thephosphor particles 13 and any air or interstitial matter between thephosphor particles or crystals, the resultant light paths 14 followed bythe optical wavelength photons is quite tortured. At each suchtransition that the optical wavelength photon encounters, the refractionand reflection that occurs causes a certain loss of optical energy.Because the light paths 14 are so tortured and long, many optical energydissipating interactions occur, in both the phosphor itself and thebinder or matrix, resulting in a cummulative loss of optical outputenergy from the scintillator body 10.

FIG. 2 is a side elevational sectional view illustrating the operationof a scintillator body 10 of the present invention. Here a high energyx-ray or gamma-ray 12 is absorbed at absorption site 11 within thescintillator body 10 and here as in the prior art, multiple lower energyoptical wavelength photons are emitted in the visible or near visible(ultraviolet and infrared) regions depending upon the phosphor employed.In the present invention, however, the phosphor particles 13 areembedded in a transparent matrix material 15 which material isindex-matched to the particular phosphor employed. Because the indicesof refraction are matched to the phosphor, the phosphor/matrix boundaryis invisible or nearly invisible to the optical wavelength photons,resulting in less distorted and convoluted light paths 14. As a directconsequence of this index-matching, the optical output energy is morereadily directed to the exterior of the scintillator body for detection,than is the light output in the prior art device shown in FIG. 1. Ascintillator structure similar to that of FIG. 2 in which supportivematrix material is transparent but is not index-matched to the phosphormaterial, also will not have as great an amount of detectable opticaloutput.

As mentioned above, it is important that the phosphor material have agood luminescent efficiency, that is, it should be able to convert asmuch of the x-ray or gamma-ray input energy into optical output energyas possible. This efficiency property is desirable for scintillatorbodies in general but in particular when scintillator bodies are used incomputerized tomography and even more particularly when they are used inthe tomographic imaging of moving bodily organs, it is also importantthat the phosphor have a short afterglow. For general tomographicapplications, it is desirable that the optical output of thescintillator body decay to 0.1 percent of its peak output within 5milliseconds of the termination of the high energy excitation. Moreover,in tomographic applications involving moving bodily organs, it isdesirable that this decay to within 0.1 percent of its peak value occurwithin 1 millisecond of the termination of the high energy excitation.

A phosphor that is particularly suited to these tomographic applicationsis barium fluorochloride with a europium activator, BaFCl:Eu. Anotherimportant property of BaFCl:Eu is its relatively low index of refractionwhich is approximately 1.66 at a wavelength of approximately 4,800 A asshown in FIG. 3. Other suitable phosphor materials include calciumfluoride (CaF₂) with an index of refraction of 1.43, barium fluoride(BaF₂) with an index of refraction of 1.47, cesium fluoride (CsF₂) withan index of refraction of 1.48, zinc silicate (Zn₂ SiO₄) with an indexof refraction of 1.62, potassium iodide (KI) with an index of refractionof 1.68, and cesium iodide (CsI) with an index of refraction of 1.78;the aforementioned indices of refraction in each case are measured atthe light wavelength of the output of the corresponding phosphor. In thecase of BaFCl:Eu, Eu activator is typically present to the extent ofapproximately 1 mole percent but may be present in the range fromapproximately 0.1 mole percent to approximately 5 mole percent.

Likewise there are certain criteria that the transparent matrix materialshould possess. It should be initially noted that when the matrix isdescribed herein as being transparent, it is meant that it istransparent at the optical wavelength of interest (see FIG. 3). In theevent that one or more wavelength conversions are employed to bettermatch the optical output of the phosphor to the sensitive spectralregions of a suitable photoelectrically responsive device, then thetransparency referred to applies at only the final wavelength region.

Another important property that the matrix material should possess isthat it be capable of supporting the phosphor particles in a stableposition with respect to the boundaries of the scintillator body. If thematrix is formed from a polymer or copolymer which hardens uponprocessing, this mechanical positional stability is not a problem.However, if the transparent matrix material is and remains in liquidform, it should have an appropriate density or be otherwise capable ofholding the phosphor particles in a stable suspension.

In all embodiments of the present invention, however, the paramountproperty of the transparent matrix material is that it be selected orcaused to have an index of refraction approximately equal to that of thescintillation phosphor. For example, a single monomer substance which isnot reactive with the phosphor employed, and which has an index ofrefraction approximately equal to that of the phosphor, may be mixedwith the phosphor before polymerization thereby forming the scintillatorbody with the desired high optical output. If a single polymer substancecannot be found with the desired properties, then two monomers may beused, one having a higher index of refraction and the other having alower index of refraction. The index of refraction of the resultantcopolymerized material is controlled by the relative proportions of thetwo copolymer materials used, the resultant index of refraction beingapproximately linearly related to the amount of the copolymers present.If this method of index refraction control is limited by the inabilityof the monomers to polymerize when mixed in the proportions needed toachieve the desired index of refraction then a different monomer set isselected.

In one embodiment of the present invention, the phosphor particles aresuspended in a matrix of a low temperature inorganic glass such as theoxides of silicon, aluminum, lithium, boron, and phosphorous, all ofwhich are low Z materials and highly non-absorptive of x-radiation.

In another embodiment of the present invention, the phosphor particlesare suspended in a liquid solution. For example, 1-bromonaphthalene hasan index of refraction close to that of BaFCl:Eu and is useful as atransparent supportive matrix material. However, 1-bromonaphthalene doeshave an index of refraction slightly lower than that of BaFCl:Eu, but1-bromonaphthalene may be mixed with methylene iodide which has an indexof refraction of approximately 1.74, the use of which in appropriateamounts permits a much closer index-matching to this particularphosphor. An appropriate solution of 1-iodonaphthalene (index ofrefraction approximately 1.68) and styrene (index of refractionapproximately 1.55) may also be used as a transparent supportive matrixmaterial for the BaFCl:Eu phosphor. Moreover, entire sets of liquids(non-polymers) are commercially available with various indices ofrefraction which can be mixed pair-wise one with another to produceliquids of any desired index of refraction. Such liquids are available,for example, from R. P. Cargill Laboratories, Inc., of Cedar Grove, N.J.

In accordance with one embodiment of the invention, the phosphor ismixed with two monomers to be polymerized. The mixture is then heated toachieve the polymerization. For example, BaFCl:Eu is mixed with 2-vinylnaphthalene and vinyl toluene and heated under vacuum at a temperaturebetween 60° C., which is the melting point of the 2-vinyl naphthalene,and 125° C. If desired, prior to thermal polymerization, the mixture iscentrifuged to achieve a greater phosphor particle density in one regionof the mixture volume. Upon polymerization, the region of greaterphosphor particle density results in a superior scintillator material.In the phosphor monomer mixture just described, settling by gravityalone produces a 35 percent volume utilization by the phosphor but ifcentrifuging is performed prior to polymerization, a 50 percent volumeutilization by the phosphor is produced. This difference in phosphordensity also produces a change in the x-ray absorption coefficient for60 kev x-rays. In particular, the 50 percent volume utilization resultsin a coefficient of 1.40 per mm, and the 35 percent volume utilizationresults in a coefficient of 0.98 per mm.

It is not necessary, however, that the phosphor be mixed initially withthe monomer or monomers involved. For example, if the phosphor chosen isreactive with any of the monomers, a different process is utilizedbeginning with the polymer instead of the monomers. Accordingly, inanother embodiment of this invention, the copolymer and any wavelengthconversion dyes, if desired, are dissolved in a solvent, such asbenzene. To this solution, the phosphor is added and mixed thoroughly.This mixture is then freeze-dried to remove the solvent and to produce ahomogeneous powder of phosphor particles encapsulated in the copolymer.This powder is then ground to break up any large aggregates of particlesand mixed to insure a homogeneous particle size distribution throughoutthe sample. This powder is then heated to or slightly above thesoftening point (glass transition temperature) of the plastic copolymerand a sufficient pressure is provided to cause the copolymer surroundingthe phosphor particles to flow, transforming the material into a singlesolid body with phosphor particles suspended therein. By way of example,for the situation in which the phosphor selected is BaFCl:Eu and themonomers are vinyl toluene and 2-vinyl naphthalene, the final vinyltoluene/vinyl naphthalene copolymer matrix is first formed and it isthis that is dissolved in the benzene. For these particular materials,the softening point for the copolymer is between approximately 125° C.and approximately 180° C. and a suitable pressure for causing thismaterial to flow is between approximately 10,000 and approximately15,000 pounds per square inch. The temperature must not be permitted torise so high as to cause decomposition of any component.

It is to be noted, that as used herein the term "polymer" also includescopolymers formed from a plurality of monomers and is not justapplicable to the situation in which a polymer is formed from a singlemonomer.

In still another embodiment of the present invention, it is possible touse the polymerized monomer or monomers rather than mixing the phosphorwith the monomer before polymerization. In this embodiment, the polymeror copolymer is preground in a suitable mill with fluorescent dyesincorporated, if desired. This powder is uniformly mixed with powderedphosphor material and this mixture is then heated to the softening pointwith sufficient pressure to cause the copolymer to flow. This processalso results in a scintillator body with superior optical output.

In those situations where the optical output of the phosphor materialdoes not match the sensitive ranges of the photoelectrically responsivedetectors, it is desirable to incorporate within or around thescintillator body wavelength conversion material or materials whichabsorb photons at the wavelength of the light output of the phosphormaterial and emit photons at a different wavelength closer to thespectral region in which the photoelectrically responsive detector ismost sensitive. The conversion efficiency of many of the fluorescentdyes that are used as wavelength conversion materials is extremely high,most of them ranging between an efficiency ratio of 94 to 100 percent.In appropriate circumstances, multiple fluorescent dyes may be providedto produce several wavelength conversions in order that the scintillatoroutput is optimally matched to the light detection means. For example,wavelength conversion substances are typically used in those cases inwhich the light output of a phosphor is in the blue to ultravioletregion of the spectrum and the detection means is a photodiode which isoptimally responsive in the red to orange region of the spectrum.

In accordance with the embodiments of the present invention, there areseveral locations in which these wavelength conversion materials areused. First, the wavelength conversion substance may be added, ifdesired, in a jacket surrounding the scintillator body such as in thestructure described in FIG. 3 of patent application Ser. No. 853,086,filed Nov. 21, 1977, by Cusano et al, now allowed. Second, in accordancewith one embodiment of the present invention, the wavelength conversionmaterial is mixed with the monomer before the scintillating phosphor isadded. Third, in accordance with another embodiment of the invention,the wavelength conversion substance is incorporated in the polymer orcopolymer prior to pulverizing the polymer or copolymer. Fourth, awavelength conversion substance may be applied as a coating on thephotoelectric detector; for example, magnesium germanate doped withmanganese (Mg₂ GeO₄ :Mn) is typically added as a photodiode coatingsince it is not readily soluable in plastic and is itself an absorber ofx-rays; however, Mg₂ GeO₄ :Mn emits light in the red to orange region ofthe spectrum to which photodiode detectors are particularly sensitive.

By way of example, when the scintillator phosphor of choice is BaFCl:Euwhich has an optical output peak at approximately 3,850 A, a two-stepwavelength conversion is accomplished by the addition of two fluorescentdyes to shift the optical output toward the red-orange region of thespectrum for more optimal detection by photodiodes. In particular, thefirst fluorescent dye employed is p-bis[2-(4-methyl-5-phenyloxazolyl)]benzene, more simply known as "dimethyl POPOP". This first dye shiftsthe optical output to approximately 4,250 A. A second dye, perylene isutilized, further shifting the radiation to approximately 4,680 A. Analternative choice for the second fluorescent dye to be added is 9,10bis(phenylethynyl) antracene (BPEA), which is utilized to shift thewavelength to approximately 5,000 A with an efficiency of between 95percent and 100 percent. The efficiency of the dimethyl POPOP itself isapproximately 95 percent and the efficiency of the perylene isapproximately 94 percent. These high efficiencies in a double wavelengthconversion process therefore result in net degradation in overallefficiency of no more than a factor of 0.80, which is more than offsetby the increased sensitivity of a photodiode type detector. All of thedyes mentioned in this example are of an aromatic nature and aretherefore soluble in and compatible with the monomers (vinyl toluene andvinyl naphthalene) described above. However, other suitable dyes may beemployed and, like the ones mentioned in the example, may beincorporated within the scintillator body or incorporated within jacketssurrounding the scintillator body. These other dyes include rhodamine-Bwith an efficiency of approximately 95 percent and BPEA also with anefficiency of approximately 95 percent. The principal criteria for theselection of these dyes, other than the particular wavelength shiftwhich they provide, is that they be efficient and highly absorptive ofemitted radiation.

By way of further example, a scintillator body is prepared by mixing 10grams of 2-vinyl naphthalene with 3 grams of vinyl toluene. To thesemonomers is added 63 milligrams of dimethyl POPOP and 31 milligrams ofperylene. This mixture is then introduced into a vessel containing 8grams of BaFCl:Eu powder and the entire mixture is thermally polymerizedunder vacuum at a temperature between approximately 60° C. and 125° C.If desired, before polymerization, the mixture is centrifuged toincrease the density of the BaFCl:Eu phosphor in one region of themixture volume.

While the above invention has been particularly described in terms ofthe BaFCl:Eu phosphor and in terms of computerized tomographicapplications, the invention is not so limited. For example, it isapplicable in industrial applications where higher energy gammaradiation is employed and is in general applicable wherever increasedoptical output is desired from a scintillator structure.

While this invention has been described with reference to particularembodiments and examples, other modifications and variations will occurto those skilled in the art in view of the above teachings. Accordingly,it should be understood that within the scope of the appended claims,the invention may be practiced otherwise than is specifically described.

The invention claimed is:
 1. A method of producing scintillator bodiesfor use in computerized tomography with increased detectable opticaloutput from a phosphor which absorbs electromagnetic radiation atsupra-optical frequencies and emits electromagnetic radiation at opticalfrequencies, said method comprising the steps of:(A) dissolving in asuitable solvent a polyvinyl naphthalene toluene polymer having an indexof refraction equal to the index of refraction of BaFCl:Eu phosphor, atapproximately the wavelength of the optical emission of the phosphor,said polymer being substantially transparent to the optical wavelengthradiation emitted by the phosphor and also being substantiallytransparent to supra-optical electromagnetic radiation; (B) adding thephosphor to the solution in step A; (C) freeze-drying the solution fromstep B, whereby the solvent is removed; (D) pulverizing the freeze-driedmaterial from step C, to form a powder with an approximately homogeneousparticle size distribution; (E) heating the powder from step D at asufficiently high temperature and pressure to cause the polymer to flowforming a solid body, but below a temperature so high as to causedecomposition of any component.
 2. The method of claim 1 in which, instep D, the pressure applied is between approximately 10,000 andapproximately 15,000 pounds per square inch and the temperature isbetween approximately 125° C. and approximately 180° C.
 3. The method ofclaim 1 in which at least one wavelength conversion substance is addedto the solution in step A.
 4. The method of claim 3 in which p-bis[2-(4methyl-5-phenyloxazolyl)] benzene is included as a first wavelengthconversion material and a fluorescent dye, selected from the groupconsisting of perylene and 9,10 bis (phenylethynyl) anthracene, isincluded as a second wavelength conversion material.
 5. The method ofclaim 3 in which the wavelength conversion substance is selected fromthe group consisting of rhodamine-B and 9,10 bis(phenylethynyl)anthracene.
 6. The scintillator body produced in accordance with claim1.